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12                                CHEMICAL ENGINEERING
The Stirling boiler has four transverse drums, one a bottom mud drum, with bent tubes expanded directly into the riveted sheet metal drums.
All of these boilers lend themselves to flexibility of arrangement. The length of tubes, numbers hi a row (horizontally and vertically), etc., may be varied to suit economical or other conditions. Boilers of this type are generally supported independently of brickwork. Standard methods of cleaning have been developed, but a large number of joints must be made after Babcock & Wilcox or Heine boilers have been completely opened for cleaning.
Boiler Steels are open-hearth. Flange or boiler steel, for general use, should have T.S. = 55,000 to 65,000 Ib. and a yield point half this, with 25 per cent elongation in 8 in. Firebox steel, for parts exposed to fire; T.S. 52,000 to 63,000 Ib., yield point half T.S., elongation 26 per cent in 8 in. Extra soft steel for rivets, T.S. 45,000 to 55,000 Ib., yield point one-half T.S., elongation 28 per cent. Steel for stays and staybolts, T.S. under 62,000, yield point one-half T.S., elongation 28 per cent.
Grates.Stationary grates may have longitudinal (common bar) or round (sawdust) openings, or may in plan have singly diagonal (Tupper) or doubly-diagonal (herringbone) apertures. All are cast iron, with metal 3 to 4 in. deep at center and M in. wide at top. Lengths of bar should not exceed 3 ft. The width of air space varies from y in. for hard coal screenings to 1 in. for bituminous lump. The percentage of air space to total area may be as much as 70 with soft coal. Diagonal openings discourage warping but make slicing difficult. Grates should pitch slightly to the rear. Depths of 6 ft. are the maximum for convenience in hand-firing, but grates 10 ft. deep are sometimes used.
Shaking grates permit of cleaning the fire while the doors are closed. The grate should be subdivided so that a portion only need be operated at one time. Both shaking and dumping should be provided for.
Boiler Settings.A good setting should be airtight and remain so: the repair cost should be moderate. These aims depend mainly on good workmanship, especially on firebrick arches. Independent support of the boiler proper from a structural framework is preferable to the common method of using the brickwork to carry the (horizontal tubular) boiler. In this method, lugs are riveted to the boiler shell. The front lugs rest on plates embedded in the setting walls, while the rear lugs rest on rollers which lie on plates built in the walls; but the walls at this point are built around to clear the lugs. A newly built or repaired setting should be dried slowly and thoroughly before use. A complete lining of firebrick is best. Every fifth firebrick course should be made of headers. These bricks should be kept dry until used, then laid in thin fireclay. Very hard firebrick should be avoided. The best are said to be those made from Pennsylvania clay.
The "back end" of a horizontal tubular boiler ^space between rear tube sheet and inside of rear wall) should be 16 to 24 in., carefully arched over, with a support making a tight joint at the head. With the orthodox type of setting (boiler carried on walls) sound construction requires two 13-in. walls with a 2-in. air space between them. When boilers are set in a battery the 28-in. wall thus formed may serve for two adjoining boilers. Side walls are tied together by bottom and top rods passing through vertical buckstaves.
Steel casings around the entire brickwork have been found so to reduce air-infiltration and radiation as to increase efficiency from 2 to 10 per cent.
Grates should be 24 in. above the floor with horizontal tubular boilers, the shell